I have complained to AT&T regarding them blocking Yahoo!Messenger. It shouldn't be that way. Their excuse was that some contracts had not been negotiated yet.
I've been using Y!M on my Curve 8310 and previous 8300 using the workaround for about 2 months. It works great! The issue is that AT&T doesn't provide the service books for Y!M for whatever their excuse is, so you have to load them manually. If you follow the directions listed in the previous post it should work.